Ink jet printing involves ejecting ink droplets from orifices in a print head onto a receiving substrate to form an image. The image is made up of a grid-like pattern of potential drop locations, commonly referred to as pixels. The resolution of the image is expressed by the number of ink drops or dots per inch (dpi), with common resolutions being 300 dpi and 600 dpi.
Ink-jet printing systems commonly utilize either direct printing or offset printing architecture. In a typical direct printing system, ink is ejected from jets in the print head directly onto the final receiving substrate. In an offset printing system, an intermediate transfer surface, such as a liquid layer, is applied to a support surface, such as a drum. The print head jets the ink onto the intermediate transfer surface to form an ink image thereon. Once the ink image has been fully deposited, the final receiving substrate is then brought into contact with the intermediate transfer surface and the ink image is transferred and fused to the final receiving substrate.
U.S. Pat. No. 5,389,958 entitled IMAGING PROCESS and assigned to the assignee of the present application is an example of an indirect or offset printing architecture that utilizes phase change ink. The intermediate transfer surface is applied by a wicking pad that is housed within an applicator apparatus. Prior to imaging, the applicator is raised into contact with the rotating drum to apply or replenish the liquid intermediate transfer surface.
Once the liquid intermediate transfer surface has been applied, the applicator is retracted and the print head ejects drops of ink to form the ink image on the liquid intermediate transfer surface. The ink is applied in molten form, having been melted from its solid state form. The ink image solidifies on the liquid intermediate transfer surface by cooling to a malleable solid intermediate state as the drum continues to rotate. When the imaging has been completed, a transfer roller is moved into contact with the drum to form a pressurized transfer nip between the roller and the curved surface of the intermediate transfer surface/drum. A final receiving substrate, such as a sheet of media, is then fed into the transfer nip and the ink image is transferred and fixed (transfixed) to the final receiving surface by the pressure exerted on it in the nip.
One constraint with the architecture taught in the '958 patent is that each of the steps recited above must be performed in series, one after another. This greatly increases the time required to complete the printing process, and also limits the maximum length of an image to approximately the circumference of the drum. Additionally, the rotational speed of the drum during the transfix process must be considerably slower than the speed of the drum during the imaging process in order to fully transfer the ink image to the final receiving surface.
With regard to the imaging process, in many direct and offset printing systems the print head and the final receiving substrate or the intermediate transfer surface move relative to one another in two dimensions as the print head jets are fired. Typically, the print head is translated along an X-axis in a direction perpendicular to media travel (Y-axis). The final receiving substrate/intermediate transfer surface is moved past the print head along the Y-axis. In this manner, the print head "scans" over the medium/substrate and forms a dot-matrix image by selectively depositing ink drops at specific pixel locations. An example of this type of imaging process is disclosed in U.S. Pat. No. 5,949,452, entitled IMAGE DEPOSITION METHOD and assigned to the assignee of the present application. The '452 patent discloses an imaging process that utilizes multiple revolutions of the drum to deposit the complete ink image on the intermediate transfer surface. In the preferred embodiment, the drum rotates through 28 revolutions as the print head moves in an X - axis direction perpendicular to the drum travel direction to deposit the image. As with the architecture taught in the '958 patent, the maximum length of a given image is limited to the circumference of the drum.
To increase image density and allow for greater speeds, multiple print heads may be utilized. It is also known to utilize one or more stationary print heads to eliminate the necessity of scanning across the transfer surface or media. An example of a multiple stationary print head printer is disclosed in U.S. Pat. 5,677,719 entitled MULTIPLE PRINT HEAD INK JET PRINTER. FIG. 8 of the '719 patent shows an alternate embodiment suitable for color printing. Four separate ink jets are utilized, with each of the ink jets assigned to one of the four primary colors magenta, cyan, yellow, and black. To overlay two primary colors to achieve a secondary color, the '719 patent requires multiple revolutions of the drum. One color is applied during one rotation and another color is overlayed on the next rotation. In this manner, multiple revolutions of the drum are required to form a complete, solid fill full-color ink image on the intermediate transfer surface. It follows that the maximum length of a given image is limited to the circumference of the drum.
The '719 patent is directed to reducing the drying time of an ink droplet on the surface of the drum. More specifically, the '719 patent addresses the drying time required for an aqueous-based ink droplet to be cleanly transferred to the final receiving surface. A drying time of three seconds is disclosed, which translates to a maximum drum rotation speed of 20 revolutions per minute, corresponding to a maximum printing speed of 20 pages per minute.
As the above description illustrates, the speed of the printing architecture disclosed in the '719 patent is limited by the required drying times and the use of multiple drum revolutions for full-color printing. The maximum length of an image is also limited to the circumference of the drum. Thus, a need remains for a high speed ink jet printing system that overcomes the drawbacks of the prior art.